He passed his medical on Saturday - having been linked with a move last week - and could make his United debut against Cardiff at Old Trafford on Tuesday.

Mata told MUTV: "Today is a very happy day for me, I'm very proud to be here. It's amazing knowing that a club like United is interested in me.

"It's a massive club. I thought 'wow' when I heard of the interest. I know this is a big money deal, I will try my best. The squad is very good, the manager is very good, the fans are unbelievable so we have everything to succeed.

"To be honest [the transfer] is a bit surprising but I have to say that I'm really grateful to Chelsea, the owner, to people working at the club and the fans, they voted me the best player of the last two seasons, and I will always be grateful to them. The last six months were difficult for me, I didn't play as much as I wanted to, I understand that football is a team sport so I respected that situation but now with this position, the chance to come to United was a great moment for me to carry on with my career and to enjoy another massive club.

"David De Gea was texting me the last 5/6 days, 'when are you coming, when are you landing, are you coming by helicopter or bike!' At the end I am here, I am very happy to be here with him, because he is a very good friend, he is an amazing goalkeeper and top guy and he was important for me arriving here.

"It will be amazing for me to play with Wayne Rooney, RVP [Robin van Persie], Adnan [Januzaj], [Antonio] Valencia, Chicharito, [Danny] Welbeck, all of them, they are amazing, they are top level.

"My best position, first of all playing. Playing, because it's what I love, to play, and to smell the turf. But after I think I can play in the three positions behind the striker. I've managed to play on the left when I arrived at Chelsea, this season I play on the right, last season which probably was the best season of my career I played behind the striker but it doesn't matter as long as I play and the manager wants me to play.

"I would like to win the Premier League here, it's a very special trophy for every player who plays in this country and it would be amazing, and I think at this club you can achieve it because this club is used to winning leagues. Obviously I'm still young and I've been lucky to win some trophies but I want to win lots more.

"Old Trafford is an unbelievable place, unbelievable stadium to play in. I have been in here two, three, four times, always with rivals, Valencia and Chelsea and now I have the chance to play at home so it's going to be very difficult but I think the fans in the stadium are incredible."

United moved for Mata earlier this week, while Chelsea manager Jose Mourinho has also sold winger Kevin De Bruyne and signed midfielder Nemanja Matic in this transfer window.

Mata scored 20 goals for Chelsea last season but has only made 11 league starts in the current campaign and has not featured since the 3-0 win over Southampton on New Year's Day. He has scored 33 goals in 133 games for Chelsea in all competitions.

United boss David Moyes added: "Juan is one of the finest playmakers in the game today and it's a real pleasure to have secured his signature. He's been instrumental in Chelsea's recent success, with a notable 28 assists and 32 goals in his last two Premier League seasons.

"I have been impressed with the excitement he has shown to be part of the new Manchester United. I know our fans have admired Juan for some time now - he has always played well against us - and I know they will be delighted to see him in the red of United, starting on Tuesday."